The present invention relates to plasma processing systems, and particularly to the delivery of RF power to initiate and sustain a plasma in such systems.
In the semiconductor fabrication industry, capacitively coupled plasma sources are widely used for dry etching and plasma enhanced chemical deposition. Dry etching is a process for removing a layer of material from a wafer surface. This removal is a result of combined mechanical and chemical effects of high-energy plasma ions striking the wafer surface. In plasma enhanced chemical deposition, a layer of a material is deposited on the wafer surface. This material is introduced into the plasma either by sputtering a target made of the material or by supplying a gas which contains the material or from which the material is produced by a chemical reaction. The material may be ionized by the plasma and can then be attracted to the wafer by an electric field.
The trend in the semiconductor fabrication industry has been toward integrated circuits having ever smaller elemental features. As a result, etch and deposition rate uniformity over the wafer surface has become more important, particularly when a layer is being etched or deposited according to a pattern. At the same time, recent developments in plasma source technology have led to the increased use of very high frequency RF excitation, e.g. from 60 to 300 MHz, and possibly even higher, to initiate and sustain the plasma.
The use of these very high excitation frequencies provides a benefit in the form of increased power coupling to the plasma, and thus excitation efficiency, that is likely caused by an increase of plasma electron temperature. This increase of RF power coupling and thus plasma density, has the effect of increasing harmonic generation in the plasma. However, maintaining high etch and deposition rate uniformity levels at these very high excitation frequencies and with strong harmonics present has proven to be a difficult feat, for a number of reasons.
For example, as the plasma RF excitation frequency is increased, the wavelength of the RF wave decreases. Thus, RF electromagnetic field spatial variations are more pronounced at these higher frequencies and this adversely affects process uniformity. In addition, another trend in the industry is to process ever larger wafers, 300 mm diameter wafer technology presently being implemented. Of course, as wafer diameter increases, the wavelength-to-wafer-diameter ratio decreases.
A plasma acts as a nonlinear RF circuit element and thus acts as a source of harmonics of the fundamental excitation frequency. These harmonics, due to their higher frequencies, have an even higher power coupling efficiency to the plasma than the fundamental. Therefore, harmonics, even if present at very low power levels, can significantly affect process uniformity due to their very unfavorable wavelength-to-wafer-diameter ratio.
Since harmonics of the RF fundamental excitation frequency have comparatively short wavelengths, they are far more likely to set up resonances in various places in the process chamber, RF transmission lines, cavities, etc., since their half-wavelengths are comparable to the dimensions of these places.
The situation is further worsened by the use of components made of high permittivity (∈) and/or permeability (xcexc) materials, or by the presence of RF transmission structural features that have significant series inductance (L) and/or shunt capacitance (C). Both of these effects reduce the wavelength of the propagating electromagnetic wave in a structure, the former by directly changing the wave propagation velocity, the latter by creating a xe2x80x9cslow-wavexe2x80x9d structure. This wavelength reduction allows harmonics to resonate in places where they normally would not be able to.
It can thus be seen that reduction of the power content of the harmonics of the RF excitation frequency would improve etch or deposition uniformity. This harmonic attenuation can take place wherever a suitable impedance-matched coupling structure is present, or can be provided, to couple the harmonic power out of the plasma.
This application is a Continuation of International Application PCT/US01/40073, filed on Feb. 9, 2001 and claims the benefit of U.S. Provisional Application 60/182,188, filed Feb. 14, 2000. In the past, various techniques have been proposed for selective attenuation of harmonic frequencies created in plasma processing systems. These techniques utilize either a low-pass filter or a trap circuit. Examples of Patents disclosing such techniques include U.S. Pat. No. 5,302,882 entitled LOW-PASS FILTER FOR PLASMA DISCHARGE, and U.S. Pat. No. 5,325,019 entitled CONTROL OF PLASMA PROCESS BY USE OF HARMONIC FREQUENCY COMPONENTS OF VOLTAGE AND CURRENT.
It is a primary object of the present invention to reduce the power levels of harmonics of the fundamental frequency of the RF excitation power in plasma processing systems.
The above and other objects are achieved, according to the present invention, by a plasma processing system composed of a chamber enclosing a plasma region, a source of RF power having a fundamental frequency and means for transmitting the RF power from the source into the plasma region for establishing an RF electromagnetic field which interacts with a gas in the plasma region to create a plasma. An energy absorbing member that includes a body of an RF absorber material is disposed in energy-receiving communication with the plasma region. The RF absorber material has a frequency dependent attenuation characteristic such that the RF absorber material attenuates electrical energy appearing in the plasma at frequencies higher than the fundamental frequency more strongly than energy at the fundamental frequency.
Objects according to the invention are also achieved by method for maintaining a plasma in a plasma region, which method includes supplying RF power at a fundamental frequency to the plasma region together with a gas in order to create an RF electromagnetic field which interacts with the gas to create a plasma that contains electromagnetic energy components at frequencies that are harmonics of the fundamental frequency, and removing those components from the plasma, wherein the step of removing is carried out by placing a body of an RF absorber material in energy-receiving communication with the plasma, the body having a frequency dependent attenuation characteristic such that the body attenuates electrical energy appearing in the plasma at frequencies higher than the fundamental frequency more strongly than energy at the fundamental frequency.